Hi, I always make a drawing (artists would call it a "cartoon") and work from that. You can get a nifty little thing called a reducing glass at quilting shops. It's the opposite of a magnifying glass- when you look through it, you get a small image of the thing you're looking at. This can help you judge the proportions and contrast in your cartoon - if you're having trouble with contrast, just color in your cartoon with your proposed colors, then look at it through the glass. I often take a drawing and scan it in so I can print out copies to try out in different colors.
Outlining is harder. A trick I use a lot with kaleidoscopes and translucent canes is to always outline the inner elements with a sandwich of color - for example, a white-black-white sandwich sheet - instead of a single color. For flowers and leaves I almost always wrap the finished cane with gold, pearl, or 50/50 pearl/silver. I think it looks more natural than opaque clay. I use a very thin layer to wrap the outside of the cane, much thinner than I use for outlining in the body of the cane, because unless you are better than I am at making super-thin slices, when you take your flower or leaf slice and press it down into the base clay, the outside edging will expand and take up more area than it did before you pressed it down. So for example if I'm making a leaf cane, and I use a #4 sheet of clay for the veins, I would wrap it in a #6 or even #7 layer. To see this effect, try making a simple spiral with a #1 translucent and a #4 white, rolling with the white layer up, and then wrap the finished cane in a #6 or #7 sheet of white. Cut slices and apply to a dark background - as you press the slices in, you'll see the outer layer "expand."
HTH - Beth CUrran